Catalytic converter systems used for cleaning exhaust gases of internal combustion engines frequently include a pre-catalytic converter placed proximate to the engine and a downstream main catalytic converter. NOx storage catalytic converters are increasingly employed as main catalytic converters in gasoline engines, which operate at least temporally in a lean operating mode. NOx storage catalytic converters, unlike conventional three-way catalytic converters, are capable of converting nitric oxides (NOx) even when the composition of the fuel mixture is super-stoichiometric (i.e., λ>1). When operating under lean conditions with excess air, the nitric oxides are stored on a storage component of the catalytic converter by forming nitrates. During periodic short regeneration phases, where the engine is operated sub-stoichiometrically, i.e., at λ≦1, the nitric oxides are desorbed and converted on a precious metal component of the NOx storage catalytic converter to compounds that are less hazardous to the environment. The operating temperature window of NOx storage catalytic converters, where NOx is stored and converted under lean exhaust gas conditions, is typically between 250 and 550° C.
Sulfur contained in the fuel and the resulting combustion product SO2 slowly deactivate the NOx storage catalytic converter under typical driving conditions. The SOx is deposited on the converter elements in the form of relatively stable sulfates which are freed up in the catalytic converter only at temperatures above approximately 650° C. This so-called sulfur poisoning increasingly blocks the storage locations of the NOx storage catalytic converter which are then no longer available for storing NOx. The amount of nitric oxides, which can be maximally stored under lean operating conditions, and the conversion rate then also gradually decrease. The time intervals during which the engine can operate under lean conditions before a NOx regeneration is required become shorter with increasing sulfur loading, which causes an increased fuel consumption. If a very large quantity of sulfur is stored, then the lean operating mode must be partially, sometimes even entirely, suppressed in favor of a stoichiometric operating mode to prevent the emission of large quantities of nitric oxide. Sulfur regeneration of the NOx storage catalytic converter can be performed from time to time to counteract an exceedingly large decrease in the NOx storage capacity due to sulfur poisoning, whereby alternatingly rich and lean exhaust gases are introduced into the catalytic converter at temperatures above its desulphurization temperature (for example 650° C.) for an exhaust gas having a lambda value of λ≦1. If the vehicle is operated under a partial load or a full load at higher RPM and engine loads, for example on divided highways or freeways, then the aforementioned desulphurization parameters, in particular the desulphurization temperature, can be reached spontaneously, whereby the sulfur is removed during normal driving operation (passive desulphurization). Conversely, if the vehicle is operated over longer distances under a low load, then the temperature in the catalytic converter is typically below the desulphurization temperature, which may require an actively initiated desulphurization. In this case, special heating measures are required for the catalytic converter to reach a sufficiently high converter temperature.
For example, it is known to retard the ignition angle, wherein the ignition time of the air-fuel mixture is retarded relative to an ignition angle that produces the highest engine efficiency. Retardation of the ignition angle reduces the combustion efficiency and simultaneously increases the combustion temperature or exhaust gas temperature. The hotter exhaust gas heats the catalytic converter faster.
Another method for increasing the exhaust gas temperature relies on so-called multiple injection, which has recently been disclosed for the direct-injection, external-ignition combustion engines (e.g., WO 00/08328, EP 0 982 489 A2, WO 00/57045). The particular combustion characteristic of the multiple injection operation increases the exhaust gas temperature compared to a purely homogeneous operation. The divided injection method also increases the ignition stability and permits a particularly late ignition angle for heating the catalytic converter.
Another heating measure known in the art involves adjusting the air-fuel ratio of individual cylinders of the engine in opposite directions, which is also referred to as lambda split. In this process, several cylinders are operated with a mixture richer than a total lambda, while other cylinders are operated with a leaner mixture. Combustion in the cylinders operating in the rich mode is incomplete compared to λ=1 operation. The non-combusted components of the exhaust gas are then exothermally converted with the residual oxygen fraction of the lean-running cylinders in a downstream catalytic converter, thereby heating the catalytic converter.
Measures for heating a catalytic converter are not only required for desulphurization of NOx storage catalytic converters, but also for raising the temperature of the catalytic converters to their desired operating temperature after an engine start. In particular, pre-catalytic converters placed proximate to the engine should be brought very quickly to their respective light-off temperature to minimize a so-called pollutant leakage during the startup phase.
However, since all heating measures increase the fuel consumption, there is a need for more effective methods to heat-up catalytic converters in the shortest possible time.